1. Field of the Invention
The present invention generally relates to force sensing resistors and, more particularly, to a force sensing resistor (FSR) having a calibration element and to a method of manufacturing such an FSR.
2. Background Art
Pressure sensitive transducers (“pressure transducers”) generate a signal indicative of the amount of pressure applied to a flexible substrate. Pressure transducers may also generate a signal based on the location of pressure applied to the flexible substrate. Pressure transducers provide input for a wide variety of applications such as remote controls, game controllers, mouse pads, tactile sensors, and the like.
A construction for a pressure transducer includes one or more force sensing resistors (FSR). Various FSRs have been disclosed, such as those described in U.S. Pat. Nos. 4,314,227; 4,314,228; and 4,489,302. A typical FSR includes three parts: a (flexible or rigid) base, a spacer, and a flexible resistive substrate. Electrically conductive traces are arranged on the base typically in separated interdigitated sets. These traces may be configured in a single zone or in multiple zones to allow, for example, pointing devices as described in U.S. Pat. Nos. 5,659,334 and 5,828,363. The spacer spaces apart the flexible resistive substrate from the base. The spacer is typically a ring of material placed onto the base around the outer edge of the conductive traces. The spacer is also typically coated with an adhesive to connect the base to the flexible resistive substrate and hold the FSR together.
The flexible resistive substrate may be made of a polymer coated on its inner face with semi-conductive or resistive ink, giving force sensing properties to the FSR. U.S. Pat. Nos. 5,296,837 and 5,302,936 describe such FSR ink. FSR ink has a high humidity coefficient of resistance. When an FSR is used in changing humidity conditions, the high humidity resistance coefficient of the FSR ink limits the usefulness of the FSR to situations where only changes in force may be measured.
In pressure transducer applications, an FSR determines applied force. Such force, for example, may be the force applied by a finger to a button; the force applied to the barrel of a pen; the force applied in each direction of a four-direction pointing device; etc. In each application, the resistance of the FSR is measured and then converted to a force using a known relationship between force and resistance. This known relationship depends on the FSR geometry and on the mechanical characteristics of the actuator that contacts the FSR surface and is typically determined independently for each designed application.
A pressure transducer may include many FSRs arranged in a pattern or an array across the area of the pressure transducer. The location of an object touching the pressure transducer may be determined by which of the FSRs are activated in response to the touch. Typically, pressure transducers are manufactured in mass quantities over time. A problem is that manufacturing tolerances between the FSRs of pressure transducers manufactured over time may be +/−25%. These tolerance variances are caused by day-to-day and batch-to-batch FSR ink inconsistencies and by daily variations in manufacturing equipment setup. Thus, a need exists for an FSR that may be corrected for the effects caused by manufacturing variations and for the effects caused by temperature and humidity variations.